1. Field of the Invention
This invention relates to a method in which information is recorded and reproduced from a circular data recording medium. In the recordation cycle, a light beam is modulated with the data and is applied to a light sensitive medium. In the reproduction cycle, a light beam of lower power than the light beam which was used in the recordation cycle is applied to the light sensitive medium, such that the variation in intensity of the light beam, as it is reflected from or passed through the medium, is detected to reproduce the data.
2. Description of the Prior Art
Magnetic disks are well known in the prior art as large capacity memories. In writing binary data in such a circular data recording medium, since it is coated with ferromagnetic material, the magnetization of the disk can be inverted at selected points according to the binary data which is to be recorded. In reproducing the data as written into the recording medium, the variations in magnetic flux due to the inverse-magnetized portions of the recording medium are detected and converted into an electrical current signal. Magnetic tapes are also well known as large capacity memories, and the writing and reading cycles of such magnetic tapes are substantially the same as those cycles of the aforementioned magnetic disk. However, since sequential access must be employed to read data from a magnetic tape, it cannot be employed as a large capacity memory in which especially short access time is required.
Recently, a so-called "optical disk device", in which data is recorded and reproduced by applying a light beam to a circular data recording medium, has also been used as a memory. The optical disk device offers the large capacity of a magnetic tape device, while also realizing the fast access time of a magnetic disk device. During the recording cycle of this device, the focusing lens of a laser optical system is focussed on an opaque film formed on a disk, sublimating the opaque film partially and forming "pits" of lesser opaqueness. In this system, one pit corresponds to one bit of information, thereby creating the aformentioned high storage capacity. During the reproduction cycle, the focusing lens in the laser optical system is focused on the pits, such that the quantity of light reflected from or passed through the pits is detected and processed. The laser output is controlled during the reproduction cycle to maintain the configuration of the pits, that is, the relative power or intensity of the laser beam is not permitted to rise to the level of intensity used during the recordation cycle so that data retention is insured.
The recording and reproducing cycles of an optical disk device will now be described in greater detail in reference to the block diagram of FIG. 1 and the waveform diagrams of FIG. 2. This detailed discussion of the recording and reproducing cycles will explain the functional deficiencies of the standard optical disk device, thereby providing a basis for the present invention.
In the recordation of a binary data train such as "1 0 0 0 1 1 0 1 1", as shown in FIG. 2(a), a laser beam . from a laser beam source 1 is applied to an optical modulator 2 as shown in FIG. 1. In the optical modulator 2, the laser beam is modulated with a recording signal with a waveform as indicated in FIG. 2(b). The modulated laser beam is applied to a recording optical system 3, a beam splitter 5 and a 1/4 waveform plate 6, and is then focused upon a recording medium 8 by the lens 7, forming a recorded pit train as illustrated in FIG. 2(c).
In the reproduction mode, the laser beam source 1 emits a laser beam which is lower in power than the laser beam emitted in the recording mode. The laser beam thus emitted is not modulated, and advances along the same path as that which the laser beam advanced in the recordation mode. Since the recording medium 8 is irradiated by the reproduction laser beam, the irradiation light is converted into a reflection light beam signal, corresponding to the recording pit train. The reflection light beam signal is applied through a reproducing optical system 9 to a photodetector 10, the output of which is applied, as a reproducing signal, to an output terminal 11. The reproducing signal has a waveform as is indicated in FIG. 2(d). The reproducing signal of FIG. 2(d) is compared with a modulation signal having a waveform as shown in FIG. 2(e), resulting in a "read" data train as indicated in FIG. 2(f). In the recordation operation, the lens 7 is driven by a track controller 12 and a focus servo unit 13, such that the signal provided to photodetector 10 is stable at all times. The elements of the system of FIG. 1 are well established in the art so that the construction of the individual components is unnecessary.
In the operation of such a standard optical disk device, as described above, several deficiencies exist which impede efficient performance. In the case where a number of bits "0" appear successively, there are no adjacent signals, and therefore it is difficult to obtain a detection signal from the reproducing signal. This places a serious design constraint upon the reading circuit as a whole, in that a detection signal would have to be reintroduced for comparision purposes. This would in turn create the need for clocking circuits and other unnecessary hardware. The device as described would have a large bit error rate. Also, since one pit corresponds to one bit, as described above, it is necessary to provide a distance substantially equal to a pit length L between pits (see FIGS. 2(b) and 2(c). Accordingly, one bit data is recorded over a length which is substantially equal to twice the pit length L, and thus the upper limit of the number of bits recorded per unitary area of the recording medium, i.e., the upper limit of a bit density, is predetermined.
To increase the bit density, signal processing can be carried out as indicated in FIG. 2(b'). Specifically, the pulse width of a recording signal corresponding to a bit "1" can be made coincident with the signal period. In this case, since one bit of data is recorded over a length equal to the pit length L, the pit density is about twice that in the above-described method. However, this signal processing method also produces a design constraint. When "1" signals appear continuously, a reproduction signal as indicated in FIG. 2(d') becomes a continuous pulse signal, again posing difficulties in obtaining a detection "window" signal from the reproducing signal (FIG. 2(d')).
Accordingly, an object of this invention is to provide a novel method in which the above-described difficulties accompanying the data recording and reproducing operations of an optical disk device are eliminated. More specifically, an object of the invention is to provide an optical data recording and reproducing method in which the bit density is high, a detection "window" signal can be readily obtained from a reproducing signal, and self-locking (or forming a "window" signal according to a reproducing signal, eliminating the need for independently introducing a detection signal) can be readily achieved.